docstr: Retrieve the command to which an alias expands.
docstr_tok: ["Retrieve","the","command","to","which","an","alias","expands","."]
code: def retrieve_alias(self, name):
      """Retrieve the command to which an alias expands."""
      caller = self.get_alias(name)
      if caller:
          return caller.cmd
      else:
          raise ValueError("%s is not an alias" % name)
  
imports: import os
  import re
  import sys
  from traitlets.config.configurable import Configurable
  from .error import UsageError
  from traitlets import List, Instance
  from logging import error
label_desc: Use image node_id 10 for calling the AliasManager obj's underlying member method code with example usage: obj.retrieve_alias(name) and returns: caller